PROJECT SUMMARY/ABSTRACT Multiple myeloma (MM) is the most common malignancy of the bone marrow (BM) and remains incurable for the vast majority of patients. It has been known for some time that cells in the host BM niche are crucial to MM growth and that interactions between MM and the host BM microenvironment can facilitate disease progression and relapse. Current treatment standards for newly diagnosed MM include high-dose melphalan with autologous stem cell transplantation followed by prolonged maintenance therapy with an immunomodulatory drug. While the effects of these agents on MM cells have been studied extensively, the effects of cytotoxic therapy on the host BM microenvironment remain poorly understood. Cytopenias, immunoparesis, and the development of acute leukemias are adverse consequences of cytotoxic therapy-induced damage to the host BM niche that have long been reported on and have a significant impact on the disease course. Intriguingly, recent research has shown that cytotoxic therapy-induced damage to the cells of the tumor-surrounding microenvironment leads to alterations that foster disease relapse. Given the intimate relationship between MM cells and the host BM niche, we hypothesize that cytotoxic therapy leads to quantitative and qualitative changes in the host BM microenvironment that can promote MM growth and progression. To address this hypothesis, we will investigate transcriptional changes in cell populations within the host BM niche before, during, and after cytotoxic therapy using single-cell RNA sequencing technology (Aim 1). The analysis of proliferation, senescence, and variation of cancer-promoting pathways within these subtypes will unravel the specific alterations induced by cytotoxic therapy. Furthermore, we will determine how cytotoxic therapy alters the regulation of immune cells and their interplay with other cells of the BM niche. Aim 2 will study how these processes differ in patients who relapse early (<2 years) and in those who relapse late (>5 years). The findings will identify the role of these immune populations in MM response to immunomodulatory drug therapy. To determine whether cytotoxic therapy- induced damage can be overcome, we will use our in-house SCID?rab mouse model (Aim 3). We will investigate the impact of inhibiting TGF-?, a cytokine that has been found to be elevated in patients after cytotoxic therapy and has been associated with tumor recurrence. Successful completion of these studies will provide a mechanistic explanation of how cytotoxic therapy-induced damage to the BM microenvironment can promote MM relapse. This will allow for the development of more effective therapies for MM, which is the long-term goal of this work.